Process for regenerating a layered double hydroxide

ABSTRACT

A process for producing alumina from bauxite is described in which bauxite is treated with an alkali to form a mixture comprising a solution of aluminum-containing ions and alumina trihydrate is precipitated from the solution. The process comprises treatment of the solution before or after the precipitation step with a layered double hydroxide in order to remove impurities from the solution by intercalation into the layers in the double hydroxide. The double hydroxide may contain layers of the formula: [LiAl 2 (OH) 6 ] + .

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.09/581,356, filed Aug. 7, 2000 now U.S. Pat. No. 6,479,024, which isincorporated by reference as if fully set forth.

PROCESS FOR PRODUCING ALUMINA

This invention relates to a process for producing alumina. Inparticular, the invention relates to a process for extracting aluminafrom bauxite.

Aluminium is produced, almost exclusively, from the alumina ore bauxiteby a combination of the Bayer and Hall-Heroult processes. In the Bayerprocess, bauxite is typically crushed, washed, dried and ground beforebeing treated with aqueous sodium hydroxide at an elevated temperature.The sodium hydroxide dissolves the alumina to form a solution from whichinsoluble impurities (“red muds”) are removed. Alumina trihydrate(Al₂O₃. 3H₂O or Al(OH)₃) as gibbsite is precipitated from the solutionand is calcined to produce alumina (Al₂O₃) for electrolytic reduction toaluminium in the Hall-Heroult process. An outline of the Bayer processis given in Chemistry and Industry, 18 Jul. 1988, p. 445–451.

The precipitation of the trihydrate from the solution (“the Bayerprocess liquor”) involves seeding the solution with crystals of thetrihydrate (in the form of gibbsite) at about 80° C. and maintaining thetemperature at this level for a significant period of time whilstcrystallisation takes place. If the trihydrate is precipitated morerapidly, for example at lower temperatures, it is found to be less pureand the impurities which it contains, such as silicates, sulphate,oxalate, carbonate, sodium and organic compounds, make it unsuitable forelectrolysis because of the adverse effects which these impurities haveon the electrolytic cell and on the resulting aluminium.

An essential feature of the Bayer process is that the liquor stream isrecycled; as a consequence, there can be a build up of contaminants,impurities and additives with time. In Bayer liquors containingrelatively high levels of oxalate as impurity, this problem is remediedto some extent by crystallising sodium oxalate from the liquor.

The contaminants in the Bayer liquor may be conveniently consideredunder three separate headings: organic impurities, inorganic impuritiesand additives.

Organic materials are introduced into the Bayer process by almost allbauxites. Whilst initially much of this is in the form of humicmaterials, with time in the liquor, degradation occurs and a vast numberof organic compounds is generated, with a wide range of molecularweights and chemical structures and functionalities. The largestconcentration of any single organic compound is the oxalate anion (C₂O₄²⁻), the penultimate degradation product of the humates present in theliquor prior to the formation of CO₂/CO₃ ²⁻. Not all of the organiccompounds have equal effects on the Bayer process. Many of the organiccompounds strongly absorb visible light, turning an otherwise colourlessliquor various shades from pale straw to virtually black. The mostserious problem associated with many of the organic compounds is theundesirable effects they have on the precipitation of gibbsite andsodium oxalate.

Certain compounds are known to be very strong poisons of gibbsiteprecipitation. For example, organic molecules containing specifichydroxyl (—OH) and carboxyl (—COOH) groups can dramatically affectgibbsite precipitation. Negative effects of these compounds can includereduction in precipitation rate, precipitation yield, product purity(both colour, and also increased sodium content, which remains in thealumina produced by calcining, and causes problems in subsequentaluminium production), morphology and strength (resistance to attrition)and an increase in the relative amount of smaller particles (fines) inthe product.

In all Bayer plants where there is an increasing concentration ofoxalate in the liquor stream with time, the sodium oxalate must beremoved, because above certain concentrations the liquor becomesunstable with respect to the solubility of sodium oxalate, and thelatter can precipitate out spontaneously, causing various problems inplant operation, including flotation and solids overflow, generation ofundesired fine particles of gibbsite and contamination of the gibbsiteproduct by co-crystallization with sodium oxalate. The controlledremoval of sodium oxalate is usually achieved by controlledprecipitation, either in a separate side stream or by deliberateco-crystallization with gibbsite, in a controlled manner, in the mainprocess stream. Alternative removal processes include “liquor burning”and adsorption onto an inert solid phase e.g., activated carbon. Some ofthe organic compounds in the Bayer liquor, apart from oxalate, canaffect the precipitation of sodium oxalate by raising its apparentsolubility (and hence lowering the driving force to make itcrystallize), thereby reducing yield, as well as precipitation rate, bypoisoning sodium oxalate crystal growth sites. The compounds which havea negative effect on the crystallization of sodium oxalate are referredto by the generic term oxalate seed poisons.

Various inorganic anions are introduced into the Bayer liquor duringplant operation, some of which build up in concentration, throughdigestion of the bauxite, from naturally occurring impurities in thebauxite and/or reagents and water, and, in the case of carbonate (CO₃²⁻), also through reaction of atmospheric carbon dioxide with thecaustic soda solution. Inorganic anions derived from impurities inbauxite include oxoanions of metals (e.g., oxoanions of transitionmetals) such as vanadate (VO₄ ³⁻), ferrate (FeO₄ ²⁻) and silicate (SiO₄⁴⁻). Other inorganic anions in the liquor include sulphate (SO₄ ²⁻) andphosphate (PO₄ ³⁻), for example.

In the case of some Bayer plants, using certain bauxites as feedstocks,the concentration of sulphate in the liquor can build up to unacceptablelevels. The problems caused by the presence of sulphate in the liquorstream include raising the ionic strength of the liquor whilst reducingthe effective level of free caustic, and potential problems of scaleformation (e.g., by gypsum, CaSO₄.2H₂O) through reaction with calciumadded during causticisation.

During the digestion stage of the Bayer process, soluble silica is takenup into solution in the Bayer liquor, and is subsequently precipitatedas de-silication product (DSP), which partially or wholly consists ofone or more sodium alumino-silicates. Whilst this precipitation stepeffectively removes the silicon from solution, it does so at the expenseof removing sodium and aluminium from useful production.

The conversion of sodium hydroxide in the Bayer liquor to sodiumcarbonate greatly reduces the effectiveness and productivity of plantoperation, and so is currently treated in many refineries by the processof causticisation, in which quicklime (calcium oxide, CaO) and/or slakedlime (calcium hydroxide, Ca(OH)₂) is added to the liquor. In practice,not all of the lime is converted to calcium carbonate, and some reactsto form other compounds, including ones which use up aluminium. Thelarge quantities of lime used, combined with the inefficiencies usuallyassociated with its use, leads to causticisation being a large expensein the operation of a Bayer plant, as well as there being associatednegative environmental impacts.

Various chemicals are added to the Bayer liquor at different parts ofthe process to achieve a range of improvements; for example, crystalgrowth modifiers and flocculants. It is sometimes advantageous to beable to remove some of these chemicals at certain stages of the process,and yet not remove others.

In view of the huge volume of bauxite which is processed around theworld, even small improvements in the process, such as a reduction inthe time taken for precipitation or a reduction in the temperature whichneeds to be maintained, can lead to vast cost savings.

Layered double hydroxides are a class of compounds which comprise twometal ions and have a layered structure. A brief review of layereddouble hydroxides is provided in Chemistry in Britain, September 1997,p. 59 to 62. The hydrotalcites, perhaps the most well-known of thelayered double hydroxides, have been studied for many years.Hydrotalcite clays useful as catalysts for hydrocarbon conversion aredisclosed in U.S. Pat. No. 5,354,932. It is known that layered doublehydroxides can intercalate certain inorganic anionic species, such ascarbonate, within the layers in their structure.

The structure of the layered hydroxides [LiAl₂(OH)₆]X, where X is Cl, Bror NO₃, and their monohydrates, has been described by Besserguenev etal., in Chem. Mater, 1997, no. 9, p. 241–247. The hydroxides can beproduced by the reaction of gibbsite [γ—Al(OH)₃] or other forms ofAl(OH)₃, such as bayerite, nordstrandite or doyleite, with lithium saltsof formula LiX. The materials can also be formed in other ways, such asby direct precipitation (see, for example, Serna et al, Clays & ClayMinerals, (1997), 25,384). The structure of the LiAl₂(OH)₆ ⁺layers inthe compounds is unusual for a layered double hydroxide since it isbased on an ordered arrangement of metal cations. The preparation ofthese compounds is also described in U.S. Pat. No. 4,348,295 and U.S.Pat. No. 4,348,297. A few other layered double hydroxides having cationordering are known, such as the compounds containing layers of formula[Ca₂Al(OH)₆]⁺.

Perrotta et al, Light Metals, 1996, pages 17 to 28 and Light Metals,1995, page 77 describe layered double hydroxide formation in Bayerliquor and its promotional effect on oxalate precipitation. The layereddouble hydroxide is formed only in situ by the addition of calcium ormagnesium hydroxide to the Bayer liquor and can reduce its oxalatecontent. Also, the same in situ layered double hydroxide formation isdescribed as enhancing oxalate precipitation from spent liquor. Sincethe layered double hydroxide is formed in situ in the Bayer liquor, withthe dissolved alumina in the liquor providing a component of thereaction in which the layered double hydroxide is formed, the liquor isnot itself treated with a layered double hydroxide. The chemistry behindthe process is not described in detail by Perrotta et al but there areindications that intercalation plays at most a minor role. A furtherapplication of the same concept is described in Perotta et al, LightMetals, 1997, 37 which involves the treatment of Bayer lake water. Thetechnology taught by Perrotta et al requires the use of relatively largeamounts of lime or magnesia which, generally, is not recycled and leadsto the removal of aluminate ions from the liquor (to form the layereddouble hydroxide) which is clearly undesirable. Also, it has thedisadvantage of potentially introducing unwanted substances into theBayer liquor; it is known, for example, that calcium ions have anadverse effect on the precipitation of gibbsite (see Cornell et al,Fourth International Alumina Quality Workshop, 2–7 Jun. 1996, pages97-). Furthermore, calcium leads to the formation of CaSO₄.2H₂O andother scale-forming compounds.

WO 97/03924 describes a method for purifying sodium aluminate solutionscontaining sodium oxalate (e.g., Bayer process liquors) in which thesodium oxalate is precipitated using tricalcium aluminate hexahydrate.The tricalcium aluminate hexahydrate acts as an initiator forprecipitation and is not described as having any effect on the otherimpurities in the liquor.

The present invention aims to alleviate or solve the problems associatedwith the presence of impurities in Bayer process liquors. The solutionto the problem involves the use of layered double hydroxides.

Accordingly, the present invention provides a process for producingalumina from bauxite which comprises treating bauxite with an alkali toform a mixture comprising a solution of aluminium-containing ions andprecipitating alumina trihydrate from the solution, characterised inthat the process comprises treating the solution before or after theprecipitation step with a layered double hydroxide in order to removeimpurities from the solution by intercalation into the layers in thedouble hydroxide. The impurities may be removed from the solution eitherpartly or substantially completely.

The term “bauxite” as used herein is intended to cover allalumina-containing ores from which alumina trihydrate (e.g., gibbsite)may be obtained.

Anionic impurities present in the solution, which may be removed in theprocess of the invention, typically include carbonate, sulphate, oxalate(derived from organics in or associated with the bauxite ore), humates,phosphates, silicates and transition metal oxoanions and it ispreferably, but not necessarily exclusively, one or more of thesespecies which are removed from the solution, either partially orcompletely, by intercalation into the double hydroxide. The process ofthe invention is particularly effective in removing carbonate and/oroxalate ions from the solution. Specific advantages of employing thedirect removal of oxalate from Bayer liquors include the fact that theconcentration of sodium oxalate in solution in any part of the Bayerplant can be kept below the equilibrium solubility value, and hence anyand all unwanted precipitations of sodium oxalate can be prevented.

The present invention can also remove the anionic organic compounds thatact as oxalate seed poisons, thereby improving the rate and yield ofsodium oxalate precipitation from Bayer liquors, and also removing thedanger inherent in operating a Bayer plant in an unstable region withrespect to sodium oxalate precipitation, when the crystallizationdriving force is delicately balanced, and slight changes in thecomposition of the organic compounds in the Bayer liquor (due to, forexample, a change in source of bauxite) can lead to unexpected andunwanted precipitation of sodium oxalate.

The present invention has advantages over the use of other agents forremoving impurities from Bayer liquor because of the selectivity of thelayered double hydroxides. It can leave in solution aluminate ions aswell as the beneficial additives, such as cationic organic species andhigh molecular weight flocculant molecules. Thus, the additional costsassociated with the non-selective removal agents, such as activatedcarbon, on account of their tendency to remove additives, can beavoided. Furthermore, the ordered arrangement of cations in layereddouble hydroxides such as those containing layers of formula LiAl₂(OH)₆⁺provides selectivity for different anions and this can be advantageoussince it may be possible to select for removal from the liquor onlythose anions which have a deleterious effect on the process.

The invention is at least partly based on the recognition that someanionic impurities may slow the rate of precipitation of the aluminatrihydrate by intercalating into the layers in the trihydrate and/oradsorbing on crystal growth surfaces and blocking active growth sites asthe trihydrate crystallises out of the solution. Furthermore, anionicimpurities can exhibit other undesirable effects; for example, sulphateions can act as templates for the crystallisation of zeolites whichcoprecipitate with the trihydrate and cause problems downstream in thealuminium production process. Oxalate ions play various roles indeleteriously affecting the process and may be considered to constitutethe most important impurity. Other impurities present in the solution,such as organic molecules and ions (e.g. humates) may also be removed inthe process of the invention.

The process of the invention is preferably based on the conventionalBayer process. The process of the invention is also able to reduce thecolour of the conventional Bayer liquor, as formed, by removal ofimpurities. The layered double hydroxide may be used to treat the Bayerliquor before gibbsite precipitation or after gibbsite precipitation(i.e., by treatment of the green (pregnant) and/or spent liquors) or atboth stages of the process. Treatment of the spent liquor reduces theimpurities which are recycled with the liquor to the start of theprocess. Treatment of the spent liquor may be carried out in addition toa conventional step of precipitating oxalate from the liquor.

The process of the invention is applicable in areas other than the Bayerprocess. Therefore, in another embodiment, the present inventionprovides a method of producing an aluminium oxide or hydroxide whichcomprises the precipitation of the oxide or hydroxide from an alkalinesolution containing aluminate ions, wherein the method comprises thestep of treating the solution with a layered double hydroxide in orderto remove impurities from the solution by intercalation into the layersin the double hydroxide. The aluminium oxide or hydroxide produced inthis way may be of high purity for speciality use.

The process of the invention is particularly effective in decolourisingBayer liquor to less coloured or colourless solutions. Therefore,another embodiment of the invention is a process for decolourisingalkaline solutions containing aluminate anions in the Bayer processwhich comprises treating the solution with a layered double hydroxide.The decolourisation may be due to the removal of coloured organic anionsand/or transition metal oxoanions from the solution.

The layered double hydroxide may be any compound which is able to removesome, or all, of the anionic impurities from the solution. The layereddouble hydroxide preferably contains aluminium as one of the metalcations in the compound and the other metal cation is preferably orderedwithin the structure of the compound. A preferred compound compriseslayers of formula [LiAl₂(OH)₆]⁺, such as [LiAl₂(OH)₆]OH, optionallyhydrated. [LiAl₂(OH)₆]OH may either be used as such or as a compound offormula [LiAl₂(OH)₆]A, optionally hydrated, wherein A is a monovalentcounterion other than OH such as fluoro, chloro, bromo or nitro, whichis converted to [LiAl₂(OH)₆]OH in situ in the alkaline solutioncontaining the dissolved alumina. However, the use of [LiAl₂(OH)₆]OH ispreferred since it releases only hydroxide ions into the liquor when animpurity is intercalated into the layers in the compound. Thus,[LiAl₂(OH)₆]OH can act to recausticise the liquor by removing anionssuch as carbonate and replacing them by hydroxide. Compounds containing[LiAl₂(OH)₆]⁺ layers are particularly advantageous since they can bereadily produced from the alumina trihydrate (gibbsite) which isprecipitated in the Bayer process and one of the starting materials forits production is therefore already available to bauxite processors inlarge quantities. Also, the use of this alumina-based compound ensuresthat any leaching of the compound has little or no contaminating effecton the solution. The [LiAl₂(OH)₆]⁺ layered compounds are stable at hightemperature (above 300° C.) and, as mentioned above, can exhibitselectivity for certain anions.

The double hydroxide may be regenerated at the end of the process. Afirst preferred method of regeneration comprises treatment withcarbonate to form a carbonate intercalate, thereby displacing theanionic impurities other than carbonate from the layers in thestructure, calcining the carbonate intercalate and hydrating theresulting product. Treatment with carbonate in this regeneration processmay involve subjecting the double layered hydroxide to an aqueoussolution of a soluble carbonate salt, such as sodium carbonate, at aboutor above room temperature for up to several hours (e.g., at 20 to 100°C. for from 1 to 48 hours). A second preferred method of regenerationinvolves treatment of the layered double hydroxide, optionally afterdisplacement of the anionic impurities other than carbonate withcarbonate (as described for the first preferred method), with an acidunder conditions which cause protonation and de-intercalation of theintercalated ions whilst leaving the layers of the layered doublehydroxide substantially intact. Suitable conditions involve treatment ofthe layered double hydroxide with dilute acid (e.g., dilute hydrochloricacid) at room temperature. Regeneration can also be carried out by othermethods, however, such as treatment of the double hydroxide with LiClsolution when [LiAl₂(OH)₆]⁺ compounds are employed.

In accordance with the conventional conditions of the Bayer process, thesolution is preferably treated with the double hydroxide when thesolution is at an elevated temperature (e.g., of about 80 to 100° C.),preferably by treatment of the spent liquor i.e., after the liquor hasbeen seeded with alumina trihydrate crystals for precipitation. Thedouble hydroxide may be used as such or on a support (e.g., of resinbeads). Treatment of the solution with the double hydroxide may involveany method of bringing the two materials together without harmfulcontamination of the solution such as, for example, passing the solutionthrough the double hydroxide using techniques well-known in filtrationand separation science. For example, treatment could take the form of abatch reaction or could involve flow of the liquor through a bed of thelayered double hydroxide.

Passing the solution through a body of the double hydroxide allows useto be made of the different intercalation properties of doublehydroxides having different guest anions. Thus, for example, it ispossible to employ a body comprising a region of formula [LiAl₂(OH)₆]OHand downstream of said region (i.e., in the direction of flow of thesolution), at least one other region of formula [LiAl₂(OH)₆]Y, wherein Yis an anion other than OH— whose presence in the solution is beneficialto the process, such that Y— ions pass into the solution onintercalation of the impurities into the layers in the double hydroxide.In this way, beneficial anions can be added to the solution in preciseamounts.

Similarly, it is possible to use, for example, a body of layered doublehydroxide comprising a region of formula [LiAl₂(OH)₆]OH and, downstreamof said region, at least one other region of formula [LiAl₂(OH)₆]Y′,wherein Y′ is an anion which intercalates less strongly into the[LiAl₂(OH)₆]⁺layers than OH—, in order that the region of formula[LiAl₂(OH)₆]Y′ may intercalate any impurities which have not alreadybeen intercalated into the region of formula [LiAl₂(OH)₆]OH. Y and Y′may be the same or different.

In another embodiment, the invention provides the use of a layereddouble hydroxide, such as [LiAl₂(OH)₆]OH optionally hydrated forexample, in the removal of anionic impurities from Bayer processliquors.

The advantages of the present invention include the following:

-   the layered double hydroxides are readily able to withstand the high    pH of the alkaline solutions used to dissolve the alumina;-   the double hydroxides may be regenerated;-   the double hydroxides can be readily produced from a material which    is already available to bauxite processors in large quantities;-   the double hydroxides can selectively remove unwanted impurities;-   the double hydroxides can recausticise the liquor by exchanging    hydroxide ions (OH—) for other ions in the liquor on intercalation;-   the intercalation process is very fast at room temperature and above    and so large volumes of solution can be treated quickly;-   the need for lime or magnesia is avoided and thus so is the    detrimental affect of calcium on the precipitation of gibbsite;-   the organic and inorganic compounds responsible for the colour of    Bayer liquor can be effectively removed;-   separation of the unwanted impurities from the solution is obtained    quickly without any substantial effect on the aluminate ions    contained in the solution and without requiring any significant    modification of the conventional Bayer process;-   alumina (e.g., gibbsite) growth rates, yield, agglomeration    behaviour, morphology, particle size distribution, purity and    strength can be improved; and-   the use of compounds based on Al(OH)₃, such as [LiAl₂(OH)₆]OH.H₂O    ensures that the double hydroxide cannot contaminate the solution.

The invention will now be illustrated by reference to the followingnon-limiting examples.

EXAMPLES Example 1 Synthesis of [LiAl₂(OH)₆]X.nH₂O(n≈1)

Al(OH)₃+LiX→[LiAl₂(OH)₆]X.nH₂OX=Cl—, Br—, OH—, NO₃—; (n≈1)Conditions: Gibbsite was stirred with at least a threefold molar excessof LiX in water (0.1M) at 90° C. for 6 hours.

Example 2 Intercalation of Anions into [LiAl₂(OH)₆]X.nH₂O

2[LiAl₂(OH)₆]Cl.nH₂O+Na₂C₂O₄→[LiAl₂(OH)₆]₂C₂O₄.nH₂O+2NaCl2[LiAl₂(OH)₆]OH.nH₂O+Na₂SO₄→[LiAl₂(OH)₆]₂SO₄.nH₂O+2NaOH

The above examples are typical reactions of this system and will occurfor most inorganic and organic anions.

Conditions: 250 mg [LiAl₂(OH)₆]Cl.nH₂O suspended in a 0.1M aqueoussolution of the required guest (threefold molar excess) and stirred atroom temperature overnight. Most reactions will actually occur in a fewseconds.

Example 3 Exchange with Carbonate

[LiAl₂(OH)₆]₂SO₄.nH₂O+Na₂CO₃→[LiAl₂(OH)₆]₂CO₃.nH₂O+Na₂SO₄Conditions: 250 mg [LiAl₂(OH)₆]₂[X].nH₂O (X=dianion) suspended in anaqueous solution of Na₂CO₃ (0.1M) and heated at 80° C. for 24 hours.

Example 4 Reformation of [LiAl₂(OH)₆]OH.nH₂O

Conditions: Calcine the carbonate intercalate at 400° C. then suspend inaqueous sodium hydroxide overnight at room temperature.This demonstrates the viability of a cyclic process for the removal ofcarbonate from Bayer liquor in which the layered double hydroxide isrecycled using the reformation procedure outlined above.

Example 5 Regeneration of [LiAl₂(OH)₆]Cl.H₂O from[LiAl₂(OH)₆]₂[CO₃].nH₂O Using Dilute Acid

150 mg of [LiAl₂(OH)₆]₂[CO₃].nH₂O was suspended in 10 ml of 1.0M HCl inethanol and stirred at room temperature overnight. X-ray diffraction ofthe resulting compound showed that it was [LiAl₂(OH)₆]Cl.H₂O though somedecomposition to gibbsite had occurred. Elemental analysis confirmedthat Cl had exchanged for CO₃ ²⁻. The chloride intercalate[LiAl₂(OH)₆]Cl.H₂O can be rapidly converted to the hydroxide intercalate[LiAl₂(OH)₆]OH.nH₂O at room temperature by treatment with excess sodiumhydroxide solution.

Example 6 Uptake of Carbonate from Alkaline Solution by [LiAl₂(OH)₆]Cl.H₂O

Conditions: 0.5 g [LiAl₂(OH)₆]Cl.nH₂O suspended at room temperature for24 hours in 10 ml of an aqueous solution containing:

-   i) 1:1 molar ratio (1M:1M) of NaOH:Na₂CO₃-   ii) 2:1 molar ratio (2M:1M) of NaOH:Na₂CO₃-   iii) 1:2 molar ratio (1M:2M) of NaOH:Na₂CO₃

The two possible products cannot be distinguished by X-ray diffractionso elemental analysis for carbon was used. Results showed that in allcases the carbon content was approximately 75% of the value obtained for[LiAl₂(OH)₆]₂CO₃nH₂O.

This example illustrates the principle of removing carbonate anions froma highly alkaline solution and therefore demonstrates the applicabilityof the technique to the removal of carbonate (and other) anions fromalkaline Bayer process liquors.

Example 7 Treatment of Bayer Liquor

Reactions were carried out using a chromatography column created fromthe layered double hydroxide [LiAl₂(OH)₆]OH.nH₂O. Aliquots of spentBayer liquor were passed down the column before it was broken intosections for analysis by XRD and elemental analysis. The XRD patternsfor all the sections showed the interlayer separation characteristic ofthe carbonate intercalate and there was a significant amount of carbonin the elemental analysis (corresponding to between 40 and 70% of thecarbon content of the pure carbonate intercalate). Analysis of the spentliquor before and after the column experiments indicated that the columnhad removed carbonate from the liquor as the total carbon content of theliquor had been reduced by 41%.

In this example, the colour of the Bayer liquor was transformed frombrown before treatment to a pale yellow colour after treatment bypassage through the column. This visual observation was confirmed by theUV/visible spectra obtained for the liquor before and after treatment.Thus, the layered double hydroxide successfully removes the relativelysmall quantities of impurities which are responsible for the colour ofthe liquor, such as some organic molecules/ions and transition metaloxoanions. The low amounts of these species means that theirintercalation cannot be detected on the basis of ordered intercalatephases in the XRD pattern.

Example 8 Intercalation of Organic Ions

Guests were obtained as either disodium or dipotassium salts. Those notavailable as salts were made by reacting the acid with a two to threefold molar excess of ethanolic NaOH, leading to the precipitation of thedisodium salt.

Anion exchange was accomplished by reacting the disodium or dipotassiumsalt of the following dicarboxylic acids, as models for the type oforganic dianions found in humates, in a three-fold molar excess with asuspension of [LiAl₂(OH)₆]Cl.H₂O in H₂O: oxalic, malonic, succinic,adipic, suberic, sebacic, fumaric, maleic, phthalic, isophthalic,terephthalic and L-malic. Typically 150 mg of [LiAl₂(OH)₆]Cl.H₂O wasreacted with a 5 ml aqueous solution of the required guest in a Young'sampoule at room temperature overnight. The solution was then filtered,washed with de-ionised water and ethanol and left to dry in air. Theintercalates were subsequently characterised by XRD, TGA and elementalanalysis. The characterising data are summarised in Table 1. Thesuberate, sebacate and isophthalate intercalates collapse on drying soXRD patterns of these compounds were recorded on a wet sample.

TABLE 1 Interlayer Spacing Elemental Analysis (%) A Stoichiometry (Å)Observed Calculated Oxalate [LiAl₂(OH)₆]₂[C₂O₄].4H₂O  8.2 C 4.92, H 4.34C 4.94, H 4.15 Li 2.39 Li 2.86 Al 21.76 Al 22.21 Malonate[LiAl₂(OH)₆]₂[C₃H₂O₄].4H₂O 10.5 C 7.13, H 4.65 C 7.21, H 4.43 Succinate[LiAl₂(OH)₆]₂[C₄H₄O₄].8H₂O 12.1 C 8.44, H 5.25 C 8.20, H 5.50 Li 2.39 Li2.37 Al 17.77 Al 18.41 Adipate [LiAl₂(OH)₆]₂[C₆H₈O₄].8H₂O 14.2 C 12.06 C12.47 H 5.48 H 5.58 Li 2.32 Li 2.26 Al 16.24 Al 17.57 Suberate[LiAl₂(OH)₆]₂[C₈H₁₂O₄].6H₂O 16.6# C 15.57, H 5.86 C 15.85, H 5.99Sebacate [LiAl₂(OH)₆]₂[C₁₀H₁₆O₄].5H₂O 18.8‡ C 19.27, H 6.19 C 19.49, H6.22 Terephthalate [LiAl₂(OH)₆]₂[C₈H₄O₄].5H₂O 14.2 C 15.49, H 4.51 C16.31, H 4.62 Al 16.94 Al 18.32 Li 2.18 Li 2.36 Isophthalate[LiAl₂(OH)₆]₂[C₈H₄O₄].4H₂O 15.1 C 15.54, H 4.40 C 16.83, H 4.41Phthalate [LiAl₂(OH)₆]₂[C₈H₄O₄].4H₂O 15.0† C 16.97, H 4.80 C 16.83, H4.41 Fumarate [LiAl₂(OH)₆]₂[C₄H₂O₄].4H₂O 12.2 C 9.90, H 3.94 C 10.01, H4.62 Maleate [LiAl₂(OH)₆]₂[C₄H₂O₄].4H₂O 12.9 C 9.26, H 3.30 C 10.01, H4.62 L-Malate [LiAl₂(OH)₆]₂[C₄H₄O₅].7H₂O 12.1 C 8.50, H 4.01 C 8.77, H4.78 Al 20.11 Al 19.69 Li 2.53 Li 2.53 ^(#)Collapses to a 15.6Å phase ondrying. ^(‡)Collapses to a 14.8Å phase on drying. ^(†)Collapses to an11.8Å phase on drying.

Example 9 Intercalation of Perrhenate into a Lithium/Aluminium LayeredDouble Hydroxide Host

The [LiAl₂(OH)₆]Cl.nH₂O host and NaReO₄ guest, as a model for thetransition metal oxoanions found in Bayer liquor, were weighed out in1:3 molar proportions corresponding to 150 mg of host and 546 mg ofguest. These were then added to an ampoule followed by 5 ml of H₂O assolvent and a stirrer bar. The contents of the ampoule were then stirredat 20° C. for approximately 48 hours. The ampoule was allowed to cooland the solution was filtered through a sintered glass frit.

X-ray diffraction and micro-analysis were performed on the solid sample.Both indicated that intercalation had occurred. X-ray diffraction showedan expanded d-spacing of 9.6Å, and micro-analysis showed that completeintercalation was occurring, giving a product of emprical formula[LiAl₂(OH)₆]ReO₄.H₂O.

100 g of [LiAl₂(OH)₆]Cl.nH₂O host would react with 121 g of NaReO₄. 100g of [LiAl₂(OH)₆]OH.nH₂O host would react with 131 g of NaReO₄.

1. A process for regenerating a layered double hydroxide which contains an anionic impurity intercalated into the layers in the layered double hydroxide which process comprises treating the layered double hydroxide with an acid under conditions which cause protonation and de-intercalation of the intercalated anionic impurity.
 2. A process according to claim 1, wherein the layered double hydroxide contains aluminum cations and alai contains other metal cations which are ordered within the structure of the layered double hydroxide.
 3. A process according to claim 2, wherein the layered double hydroxide comprises layers of formula [LiAl2(OH)6]+.
 4. A process according to claim 1, wherein the anionic impurity is an anion selected from the group consisting of sulphate, carbonate, oxalate, silicates, phosphate, vanadate, ferrate and mixtures of two or more thereof.
 5. A process according to claim 4, wherein the anionic impurity is an anion selected from the group consisting of carbonate, oxalate and a mixture thereof.
 6. A process according to claim 1, wherein the anionic impurity is selected from the group consisting of transition metal-based oxoanions and organic anions.
 7. A process according to claim 1, wherein the layered double hydroxide contains an anionic impurity other than carbonate anions and wherein, before treating with an acid, the layered double hydroxide is treated with a carbonate thereby displacing the anionic impurity from the layers in the layered double hydroxide with carbonate anions to form a carbonate intercalate.
 8. A process according to claim 1, wherein the regenerated layered double hydroxide removes anionic impurities from Bayer process liquor. 